Core java concepts

Core java concepts

• 2.
   The javadocprogram generates HTML API documentation from the “javadoc” style comments in your code. /* This kind comment can span multiple lines */ // This kind is of to the end of the line /* This kind of comment is a special * ‘javadoc’ style comment */
• 3.
   The classis the fundamental concept in JAVA (and other OOPLs)  A class describes some data object(s), and the operations (or methods) that can be applied to those objects  Every object and method in Java belongs to a class  Classes have data (fields) and code (methods) and classes (member classes or inner classes)  Static methods and fields belong to the class itself  Others belong to instances
• 4.
  class Person {Variable String name; int age; Method void birthday ( ) { age++; System.out.println (name + ' is now ' + age); } }
• 5.
  { int x= 12; /* only x available */ { int q = 96; /* both x and q available */ } /* only x available */ /* q “out of scope” */ } { int x = 12; { int x = 96; /* illegal */ } } This is ok in C/C++ but not in Java.
• 6.
   Java objectsdon’t have the same lifetimes as primitives.  When you create a Java object using new, it hangs around past the end of the scope.  Here, the scope of name s is delimited by the {}s but the String object hangs around until GC’d { String s = new String("a string"); } /* end of scope */
• 7.
   Java methodsand variables can be declared static  These exist independent of any object  This means that a Class’s ◦ static methods can be called even if no objects of that class have been created and ◦ static data is “shared” by all instances (i.e., one rvalue per class instead of one per instance class StaticTest {static int i = 47;} StaticTest st1 = new StaticTest(); StaticTest st2 = new StaticTest(); // st1.i == st2.I == 47 StaticTest.i++; // or st1.I++ or st2.I++ // st1.i == st2.I == 48
• 8.
  public class Circle{public class Circle { // A class field// A class field public static final double PI= 3.14159; // A usefulpublic static final double PI= 3.14159; // A useful constantconstant // A class method: just compute a value based on the// A class method: just compute a value based on the argumentsarguments public static double radiansToDegrees(double rads) {public static double radiansToDegrees(double rads) { return rads * 180 / PI;return rads * 180 / PI; }} // An instance field// An instance field public double r; // The radius of thepublic double r; // The radius of the circlecircle // Two methods which operate on the instance fields of// Two methods which operate on the instance fields of an objectan object public double area() { // Compute the area ofpublic double area() { // Compute the area of the circlethe circle return PI * r * r;return PI * r * r; }} public double circumference() { // Compute thepublic double circumference() { // Compute the circumference of the circlecircumference of the circle return 2 * PI * r;return 2 * PI * r; }} }}
• 9.
   Subscripts alwaysstart at 0 as in C  Subscript checking is done automatically  Certain operations are defined on arrays of objects, as for other classes ◦ e.g. myArray.length == 5
• 10.
   Person mary= new Person ( );  int myArray[ ] = new int[5];  int myArray[ ] = {1, 4, 9, 16, 25};  String languages [ ] = {"Prolog", "Java"};  Since arrays are objects they are allocated dynamically  Arrays, like all objects, are subject to garbage collection when no more references remain ◦ so fewer memory leaks ◦ Java doesn’t have pointers!
• 11.
  Example Programs
• 12.
   C:UMBC331java>type echo.java // This is the Echo example from the Sun tutorial  class echo {  public static void main(String args[]) {  for (int i=0; i < args.length; i++) {  System.out.println( args[i] );  }  }  }  C:UMBC331java>javac echo.java  C:UMBC331java>java echo this is pretty silly  this  is  pretty  silly  C:UMBC331java> NSIT ,Jetalpur
• 13.
  /* This programcomputes the factorial of a number */ public class Factorial { // Define a class public static void main(String[] args) { // The program starts here int input = Integer.parseInt(args[0]); // Get the user's input double result = factorial(input); // Compute the factorial System.out.println(result); // Print out the result } // The main() method ends here public static double factorial(int x) { // This method computes x! if (x < 0) // Check for bad input return 0.0; // if bad, return 0 double fact = 1.0; // Begin with an initial value while(x > 1) { // Loop until x equals fact = fact * x; // multiply by x each time NSIT ,Jetalpur
• 14.
   Classes shoulddefine one or more methods to create or construct instances of the class  Their name is the same as the class name ◦ note deviation from convention that methods begin with lower case  Constructors are differentiated by the number and types of their arguments ◦ An example of overloading  If you don’t define a constructor, a default one will be created.  Constructors automatically invoke the zero argument constructor of their superclass when they begin (note that this yields a recursive process!) NSIT ,Jetalpur
• 15.
   Java methodsare like C/C++ functions. General case: returnType methodName ( arg1, arg2, … argN) { methodBody } The return keyword exits a method optionally with a value int storage(String s) {return s.length() * 2;} boolean flag() { return true; } float naturalLogBase() { return 2.718f; } void nothing() { return; } void nothing2() {}